Support for contract based programming in C++

1. Introduction

This paper is based on P0380 (P0380R1[1]).
All changes are relative to latest working draft (N4618[2]).

2. Summary of wording

2.1. A new attributes syntax

To simplify the way that contracts are specified, and in line with our previous design paper,
we propose a new syntax that may be used for attributes.

We propose this new syntax to be usable by attributes taking a single argument, which is a valid expression.

[[contract-attribute: expression]]

where a contract-attribute is one of the following: expects, ensures, assert.

To avoid confusion,
only the colon syntax for attributes is permitted in contract specifications

We also needed to support the idea of assertion levels needed by contracts.
For this reason, we have introduced the concept of an attribute modifier that may appear after the attribute token itself.

[[contract-attribute modifier: expression]]

In this way we may specify contracts with assertion levels easily.
We require that each attribute using the new proposed attribute syntax explicitly lists its accepted modifiers (if any).

Finally, we needed to introduce the ability to define the return value to be used in postconditions.
We allow, that an attribute lists an identifier which is associated with the return value of a function.

Note that while assert(expression) would expand as a function-like macro
with the appropriate header, assert: is not a function-like invocation, so
does not expand.

2.2. Functions versus function types

The current standard establishes a distinction between an attribute applied to a function and to the function type
(the normative text uses the form "appertain to function type").
With that approach there would be a difference betweeen the following cases:

However, we know of no example where an attribute is effectively used to annotate the function type.
Besides, the current status makes it difficult to annotate functions with preconditions and postconditions.
Consequently, in this paper we propose that attributes at the end of a function declaration apply to the
function itself.

Consequently, we require that contracts are odr-identical in redeclarations,
allowing for argument names variation.

2.4 Structured bindings and postconditions

One might imagine using structured bindings in postconditions:

std::tuple f()
[[ensures [x,y]: x>0 && y.size()>0]];

However, we decided that this is something that might be considered for a
future version. The same effect can be achieved currently as follows:

std::tuple f()
[[ensures r: get<0>(r)>0 && get<1>(r).size()>0]];

2.5 Information of contract_violation

The information in contract_violation could be partially represented
by a source_location object, from the Library Fundamentals V2
Technical Specification.

However, we defer this decision for a future version of this proposal.

2.6 Name lookup of contracts

Name lookup resolution in contracts may interact with the use of different build modes.

There are two answers here. The first one would be to make resolution dependent on the
current build mode, requiring that only the contracts that would be evaluated in the
current build mode need to parse correctly and pass the name lookup.

A second solution would be to make name lookup independent of build mode. In that case,
all contracts would be required to pass name lookup independently of their assertion level.
We have selected this second solution.

2.7 Identical contracts

We are also requiring in the wording that a redeclaration of a function that contains
a contract needs to use the same contract (in the sense of ODR)
that was present in the first declaration of the function.

The exact meaning would be to require contracts to be lexically the same token by token.
This is a simpler definition, but would require that a redeclaration uses also the same
names for functions arguments (if/when they are used in the contract).

A second solution would be to require the contracts to be logically equivalent which seems
to introduce a number of additional implementation challenges.

A third option is to say that the functions must be textually equivalent except
for a change of (parameter) variable names, but otherwise having the same
structure. We have selected this option.

We have taken the route of requiring contract expressions to be odr-identical,
except for the change of function parameter names.

2.8 Throwing violation handler

If a violation handler throws an exception, it is necessary to clarify what is the effect
when the continuation mode is off.

One option could be that the exception propagates as the handler did not
return. However, that design option would open the opportunity for continuation
when the continuation has been set to off.

Another design alternative would be to unconditionally invoke terminate().

2.9 Additional information in contract violation

Besides original information in contract_violation, a new member
has been added to store the assertion_level. This value contains
a string with the assertion-level of the contract that has been violated.

2.10 Invoking the handler

The proposal does not support the direct invocation of the violation handler.
Allowing so, would imply access to handler supplied by the user, and could
result into a security issue.
Instead, we have added an additional assertion-level named
always. Such assertion cannot be disbled and the check is
performed even when the program is built with build-level set to
off.

This assertion level is available only for [[assert]]. A future
extension migh consider the usefulness of always for
preconditions and postconditions.

Background of always level

The addition of the always assertion level was introduced during the Kona
meeting. We reproduce here the relevant results from straw polls in the Evolution Working
Group.

Question

SF

F

N

A

SA

Accept and proceed to LEWG

13

6

5

4

4

Accept and proceed to LEWG without always

5

13

9

5

2

Also, an up-down vote was taken:

Proposal as is

Proposal wihtout always

14

9

Note

This version of the document does not include wording for the always level.

2.11 Issues to be solved

Initialization of contract_violation objects

When a contract is broken, a contract_violation needs to be created with
the corresponding information. There are several options on how such object should
be populated with information.

Among the available options, one could be to leave this details as something to be defined
by implementations. Otherwise, the exact population approach would need to be standardized.

Additional information in violation_info objects

Besides the information already defined in contract_violation, additional information
might be useful. One example of such information is the assertion level of the
contract assertion whose check generated the invocation of the violation handler.

3. Questions about contracts programming

What is the effect of violating a contract while evaluating the check for
another expression

Nothing special needs to be considered. When the first contract is violated the
corresponding action is taken.

Contract attribute as identifier

What happens if you try to use an identifier named as a contract attribute (e.g.
requires, audit, ...)?
Example:

X f(X & audit) [[ensures audit: audit.valid()]];

This is valid code. The first audit is interpreted as a
contract-level. Then, the conditional-expression identifies the
second audit correctly as the function argument.

4. Proposed Wording

Modify [5.1.5/6] as follows:

6 The closure type for a non-generic lambda-expression has a public inline function call operator (13.5.4)
whose parameters and return type are described by the lambda-expression's parameter-declaration-clause
and trailing-return-type respectively. For a generic lambda, the closure type has a public inline function call
operator member template (14.5.2) whose template-parameter-list consists of one invented type template-parameter
for each occurrence of auto in the lambda’s parameter-declaration-clause, in order of appearance.
The invented type template-parameter is a parameter pack if the corresponding parameter-declaration declares
a function parameter pack (8.3.5). The return type and function parameters of the function call operator
template are derived from the lambda-expression’s trailing-return-type and parameter-declaration-clause by
replacing each occurrence of auto in the decl-specifiers of the parameter-declaration-clause with the name of
the corresponding invented template-parameter.
[Example:

— end example ]
This function call operator or operator template is declared const (9.2.2) if and only if
the lambda-expression’s parameter-declaration-clause is not followed by mutable.
It is neither virtual nor
declared volatile. Any noexcept-specifier specified on a lambda-expression applies to the corresponding
function call operator or operator template. An attribute-specifier-seq in a lambda-declarator appertains
to the type of the corresponding function call operator or operator template. The function call operator
or any given operator template specialization is a constexpr function if either the corresponding lambda-expression’s
parameter-declaration-clause is followed by constexpr, or it satisfies the requirements for a
constexpr function (7.1.5).
[ Note:
Names referenced in the lambda-declarator are looked up in the context in
which the lambda-expression appears.
— end note ]
[ Example:

is divided into three parts. Attributes are described in 7.6. decl-specifiers, the principal components of a
decl-specifier-seq, are described in 7.1. declarators, the components of an init-declarator-list, are described
in Clause 8. The attribute-specifier-seq appertains to each of the entities declared by the declarators of the
init-declarator-list.
[ Note:
In the declaration for an entity, attributes appertaining to that entity may appear
at the start of the declaration and, after the declarator-id for that declaration
, or at the end of the function declaration.
— end note ]
[ Example:

1.
A precondition is a predicate that should hold upon entry into a function.
It expresses a function's expectation on its arguments and/or the state of objects that may be used by the function.
Preconditions are expressed by expects attributes (7.6.10).

2.
A postcondition is a predicate that should hold upon exit from a function.
It expresses the conditions that a function should ensure for the return value and/or the state
of objects that may be used by the function.
Postconditions are expressed by ensures attributes (7.6.11).

3.
An assertion is a predicate that should hold at its point in a function body.
It expresses the conditions, on objects that accessible at its point in a body, that must be satisfied.
Assertions are expressed by assert attributes (7.6.12).

4.
Preconditions, postoconditions, and assertions are collectively called contracts.
A contract shall have no observable effect in a correct program (a program where
all contracts would be satisfied, if they were evaluated).

5.
Contract attributes are followed by a conditional-expression, which is a potentially evaluated expression (3.2).
[Example

6.
A contract attribute may be combined with the following contract-levels:
default, audit, and axiom to express
the assertion-level of the contract. When no modifier is provided, the
assertion-level of the precondition is default.
[Note:
A defaultassertion-level is expected to be used for those preconditions, postconditions and assertions
where the cost of run-time checking
is assumed to be small (or at least not expensive) compared to the cost of executing the function.
An auditassertion-level is expected to be used for those preconditions, postconditions and assertions
where the cost of run-time checking
is assumed to be large (or at least significant) compared to the cost of executing the function.
An axiomassertion-level is expected to be used for those preconditions, postconditions and assertions
that are formal comments and are not evaluated at run-time.
— end note]

7.
A program with a contract expression that performs an observable modification of an object is ill-formed;
no diagnostic required.

8.
A contract of a constexpr function cannot refer to non-local objects that are not
constexpr.
[Example

9.
Every redeclaration of a function must contain exactly the same contract that was
present in the first declaration of that function or completely omit the contract.
The contracts expression shall be odr identical except for the change of
function parameter names.

10.
A translation may be performed with one of the following build levels:
off, default, or audit.
A translation with build level set to off shall not check any
contract.
A translation with build level set to default shall perform
checking for default contracts.
A translation with build level set to audit shall perform checking
for default and audit contracts.
[Note:
The mechanism for selecting the build level is implementation defined.
If no build level is selected, the build level is default.
— end note]
There shall be no programmatic way of setting, modifying or querying the build level of a translation unit.

11.
If a function has multiple preconditions or postconditions that would be checked,
their evaluation will be performed in the order they appear.

12.
A violation handler function is a function with the following signature:

void(const std::contract_violation &);

13.
If a contract violation is detected, a violation handler will be invoked.
It shall be implementation defined how the violation handler is established for a program and how the
std::contract_violation argument value is set.
There shall not be any programmatic way of setting or modifying the which violation handler
is called in the event of a contract violation.

14.
If a user-provided violation handler exits by throwing an exception and a contract
is violated in a call to a function with a non-throwing exception specification,
then the behavior is as if the exception was thrown within the function body.
[Note:
The function std::terminate() will be invoked.
No stack unwinding will be performed.
— end note]
[Example:

15.
A translation may be performed with a violation continuation mode, which can be: off or on.
A translation with a violation continuation mode set to off shall terminate execution by
invoking std::terminate() after completing the execution of the violation handler.
A translation with a violation continuation mode set to on
shall continue execution after completing the execution of the violation handler.
[Note:
If no continuation mode is selected, the default continuation mode is off.
— end note]
[Note:
A continuation mode set to on provides the opportunity to install a logging handler to instrument
a pre-existing code base and fix errors before enforcing checks.
— end note]
[Example:

3.
The expression of a precondition from a function may use the function's arguments.
The expression of a precondition from a function template or a member function of a class template may use the template arguments.
The expression of a precondition from a public member function shall not use members from the protected or private interfaces.
The expression of a precondition from a protected member function shall not use members from the private interface.
The expression of a precondition from a lambda-expression may use any entity captured implictly or explictly.
The expression of a precondition from a lambda-expression shall not use any entity that is not accessible by lambda-expression.

4.
A function pointer shall not include preconditions.
A call through a function pointer to functions with preconditions shall
perform contract assertions checking once.
[Example:

3.
The expression of a postcondition from a function may use the function's arguments.
The expression of a postcondition from a function template or a member function of a class template may use the template arguments.
The expression of a postcondition from a public member function shall not use members from the protected or private interfaces.
The expression of a postcondition from a protected member function shall not use members from the private interface.
The expression of a postcondition from a lambda-expression may use any entity captured implictly or explictly.
The expression of a postcondition from a lambda-expression shall not use any entity that is not accessible by lambda-expression.

4.
A postcondition may introduce an identifier to represent the return value of the function.
[Example:

2.
The expression of an assertion may use any object that can be accessed from the point of that assertion.

Modify clause [8.3.5/1]:

1.
In a declaration T D where D has the form

D1( parameter-declaration-clause ) cv-qualifier-seqopt

ref-qualifieropt noexcept-specifieropt attribute-specifier-seqopt
and the type of the contained declarator-id in the declaration T D1 is
“derived-declarator-type-listT”, the type
of the declarator-id in D is “derived-declarator-type-listnoexceptopt
function of (parameter-declaration-clause) cv-qualifier-seqopt ref-qualifieropt returning T”,
where the optional noexcept is present if and only if the
exception specification (15.4) is non-throwing. The optional attribute-specifier-seq appertains to the function
type.

and the type of the contained declarator-id in the declaration T D1 is
“derived-declarator-type-listT”, T shall be
the single type-specifierauto. The type of the declarator-id in D
is “derived-declarator-type-listnoexceptopt
function of (parameter-declaration-clause) cv-qualifier-seqopt ref-qualifieropt returning U”,
where U is the type
specified by the trailing-return-type, and where the optional noexcept is present if and only if the exception
specification is non-throwing. The optional attribute-specifier-seq appertains to the function type.

Add at the end of clause 10.3:

17.
An overriding function shall have exactly the same contract that was declared for that
function in the base class. However, the contract in the overriding function may be
omitted in which case it has the same contract as was specified in the base class.

Modify clause 17.5.1.3/2:

2.
A freestanding implementation has an implementation-defined set of headers. This set shall include at least
the headers shown in Table 19.

Table 19 — C++ headers for freestanding implementations

Subclause

Header(s)

><ciso646

18.2 Types

<cstddef>

18.3 Implementation properties

<cfloat><limits><climits>

18.4 Integer types

<cstdint>

18.5 Start and termination

<cstdlib>

18.6 Dynamic memory management

<new>

18.7 Type identification

<typeinfo>

18.8 Contract violation handling

<contract>

18.89 Exception handling

<exception>

18.910 Initializer lists

<initializer_list>

18.1011 Other runtime support

<cstdalign><cstdarg><cstdbool>

20.15 Type traits

<type_traits>

29 Atomics

<atomic>

Modify clause 18.1/2:

2.
The following subclauses describe common type definitions used throughout the library, characteristics of
the predefined types, functions supporting start and termination of a C++ program, support for dynamic
memory management, support for dynamic type identification, support for contract violation handling,
support for exception processing, support for
initializer lists, and other runtime support, as summarized in Table 32.

Table 32 — Language support library summary

Subclause

Header(s)

18.2 Common definitions

<cstddef><cstdlib>

18.3 Implementation properties

<limits><climits><cfloat>

18.4 Integer types

<cstdint>

18.5 Start and termination

<cstdlib>

18.6 Dynamic memory management

<new>

18.7 Type identification

<typeinfo>

18.8 Contract violation handling

<contract>

18.89 Exception handling

<exception>

18.910 Initializer lists

<initializer_list>

18.1011 Other runtime support

<csignal><csetjmp><cstdalign><cstdarg><cstdbool><cstdlib>

Add a new section after 18.8

18.8 Contract violation handling

1.
The header <contract> defines a type for reporting information
about contract violations generated by the implementation.